The present invention relates to a device for gas sensing, particularly at high ambient temperatures, with improved long-term stability and reproducibility.
It is known that catalytic metals can be used as gates for gas sensitive field-effect devices (transistors, capacitors, diodes, etc). Thus they comprise metal-insulator-semiconductor or metal-semiconductor structures. Such devices may be used to measure small concentrations of molecules like hydrogen, hydrogen sulfide, alcohols, hydrocarbons, ammonia, amines, etc. The highest operation temperature is determined by the semiconductors used, which e.g. for silicon is about 250xc2x0 C. but for silicon carbide about 1000xc2x0 C.
The gas sensitivity occurs because reaction intermediaries, e.g. hydrogen atoms, give rise to electrical polarization phenomena at the metal-insulator or metal-semiconductor interface, which changes the electric field outside the semiconductor. FIGS. 1a and 1b show, in cross section, a schematic representation of two types of prior art catalytic field-effect sensors. More complex structures with additional buffer layers between the insulator and catalytic metal or between the semiconductor and metal have been fabricated in order to obtain an increased stability of the devices at elevated temperatures. FIG. 1a illustrates a device with a thick, continuous catalytic metal layer 1 on top of an insulator 1, in turn supported by a semiconductor, while FIG. 1b illustrates a device with a thin, porous catalytic layer of film 11 that leaves part of the underlying insulator 12 or semiconductor 13 surface exposed to the ambient gas molecules. Continuous catalytic film devices illustrated in FIG. 1a are only sensitive to hydrogen-containing molecules since only hydrogen atoms can diffuse through the metal and give rise to a dipole at the metal-insulator or metal-semi interface. Porous catalytic film devices illustrated in FIG. 1b, on the other hand, are sensitive to many more compounds since also reaction intermediaries located on the regions of the insulator or semiconductor exposed to the ambient can contribute to measurable electrical polarization effects. Therefore the porous catalytic film devices are more often used in chemical sensor arrays when there is a need to detect a broad range of molecules. The nature of the porous films, however, results in two important and well-known problems. First, it is difficult to have a reproducible production of this type of device since the sensitivity to different compounds to a large degree depends on the exact distribution of the catalytic metal across the surface. The sensitivity thus depends on the exact shape and distribution of the catalytic metal grains, parameters that are very difficult to control during the fabrication process. Secondly, the long-term stability of the devices is limited, especially at elevated temperatures, since the metal layer continuously undergoes a restructuring process, resulting in a time variation of the chemical sensitivity of the devices. In fact, for very thin catalytic metal films the thermodynamic equilibrium will only be reached when isolated metal islands have been formed resulting in a complete failure of the devices. There is thus a need for more reproducible and stable porous catalytic film field-effect sensors.
The present invention discloses a gas sensitive porous catalytic film field-effect sensor offering improved reproducibility and long-term stability, especially at elevated operating temperatures. These improvements are obtained by depositing a thin catalytic layer 21 (FIG. 2) on an insulator 22 (supported by a semiconductor 23) or on a semiconductor that has a suitable and well-defined surface morphology. This morphology forces the catalytic film into a well-defined structure during the manufacturing process (thus improving the reproducibility) and prevents the metal from restructuring during operation (thus improving the long-term stability). An added advantage as compared to prior-art porous field-effect sensors is that the amount of catalytic material can be increased for a given amount of porosity. This will further improve the stability of the sensor devices since they will become less prone to poisoning effects. A typical device according to the invention is schematically illustrated in FIG. 2.
According to the first object of this invention, a field-effect gas sensor is fabricated by depositing a thin conducting layer onto a semiconducting substrate which has been given a suitable morphology by using masking, lithography, and etching techniques. The resulting device can, e.g., be operated as a Schottky-barrier device or as a tunneling device if a thin insulating layer has been added between the semiconductor and the conducting layer. The conducting layer can, e.g., consist of catalytic metals, alloys, or compounds or polymers in which case any semiconductor can be used. If a catalytically-active semiconducting substrate is used, the conducting layer could be catalytically active or inactive.
According to the second object of this invention, a field-effect gas sensor is fabricated by depositing a thin conducting layer onto a semiconducting substrate which has been given a suitable morphology by utilizing a naturally obtained morphology resulting from, e.g., an etching process or a deposition technique. The resulting device can, e.g., be operated as a Schottky-barrier device or as a tunneling device if a thin insulating layer has been added between the semiconductor and the conducting layer. The conducting layer can, e.g., consist of catalytic metals, alloys, or compounds or polymers in which case any semiconductor can be used. If a catalytically-active semiconducting substrate is used, the conducting layer could be catalytically active or inactive.
According to the third object of this invention, a field-effect gas sensor is fabricated by depositing a thin conducting layer onto an insulating layer grown on a semiconducting substrate which has been given a suitable morphology by using masking, lithography, and etching techniques. The resulting device can, e.g., be operated as a field-effect transistor or as a capacitor. The conducting layer can, e.g., consist of catalytic metals, alloys, or compounds or polymers in which case any insulator can be used. If a catalytically-active insulator is used, the conducting layer could be catalytically active or inactive.
According to the fourth object of this invention, a field-effect gas sensor is fabricated by depositing a thin conducting layer onto an insulating layer grown on a semiconducting substrate which has been given a suitable morphology by utilising a naturally obtained morphology resulting from, e.g., an etching process or a deposition technique. The resulting device can, e.g., be operated as a field-effect transistor or as a capacitor. The conducting layer can, e.g., consist of catalytic metals, alloys, or compounds or polymers in which case any insulator can be used. If a catalytically-active insulator is used, the conducting layer could be catalytically active or inactive.
According to the fifth object of this invention, a field-effect gas sensor is fabricated by depositing a thin conducting layer onto an insulating layer grown on a semiconducting substrate where the insulating layer has been given a suitable morphology by using masking, lithography, and etching techniques. The resulting device can, e.g., be operated as a field-effect transistor or as a capacitor. The conducting layer can, e.g., consist of catalytic metals, alloys, or compounds or polymers in which case any insulator can be used. If a catalytically-active insulator is used, the conducting layer could be catalytically active or inactive.
According to the sixth object of this invention, a field-effect gas sensor is fabricated by depositing a thin conducting layer onto an insulating layer grown on a semiconducting substrate where the insulating layer has been given a suitable morphology by utilising a naturally obtained morphology resulting from, e.g., an etching process or a deposition technique. The resulting device can, e.g., be operated as a field-effect transistor or as a capacitor. The conducting layer can, e.g., consist of catalytic metals, alloys, or compounds or polymers in which case any insulator can be used. If a catalytically-active insulator is used, the conducting layer could be catalytically active or inactive.